Bob bikes to school every day

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rsarashi: Master | Next Rank: 500 Posts; Posts: 186; Joined: Sat Dec 24, 2016 12:38 am; Thanked: 5 times; Followed by:3 members

Bob bikes to school every day

by rsarashi » Sun May 14, 2017 4:18 am

Bob bikes to school every day at a steady rate of x miles per hour. On a particular day, Bob had a flat tire exactly halfway to school. He immediately started walking to school at a steady pace of y miles per hour. He arrived at school exactly t hours after leaving his home. How many miles is it from the school to Bob's home?

A) (x + y) / t

B) 2(x + t) / xy

C) 2xyt / (x + y)

D) 2(x + y + t) / xy

E) x(y + t) + y(x + t)

OAC

Hi Experts ,

I was trying to solve this question by letting the value of x and y, but didn't come up with solution.

Please help.

Thanks.

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Source: Beat The GMAT — Problem Solving | Sun May 14, 2017 4:18 am

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pm; Location: New York, NY; Thanked: 13060 times; Followed by:1906 members; GMAT Score:790

by GMATGuruNY » Sun May 14, 2017 5:11 am

Bob bikes to school every day at a steady rate of x miles per hour. On a particular day, Bob had a flat tire exactly halfway to school. He immediately started walking to school at a steady pace of y miles per hour. He arrived at school exactly t hours after leaving his home. How many miles is it from the school to Bob's home?

(x + y) / t

2(x + t) / xy

2xyt / (x + y)

2(x + y + t) / xy

x(y + t) + y(x + t)

Let the distance to school = 20 miles.
Let x = 5 miles per hour.
Time spent biking = 10/5 = 2 hours.
Let y = 2 miles per hour.
Time spent walking = 10/2 = 5 hours.
Total time = t = 2+5 = 7.
The questions asks for the total distance: 20 miles. This is our target.

New we plug x=5, y=2, and t=7 into the answers to see which yields our target of 20.

Only answer choice C works:
2xyt/(x+y) = (2*5*2*7)/(5+2) = 140/7 = 20.

The correct answer is C.

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by Brent@GMATPrepNow » Sun May 14, 2017 5:30 am

rsarashi wrote:Bob bikes to school every day at a steady rate of x miles per hour. On a particular day, Bob had a flat tire exactly halfway to school. He immediately started walking to school at a steady pace of y miles per hour. He arrived at school exactly t hours after leaving his home. How many miles is it from the school to Bob's home?

A) (x + y) / t

B) 2(x + t) / xy

C) 2xyt / (x + y)

D) 2(x + y + t) / xy

E) x(y + t) + y(x + t)

These kinds of questions (Variables in the Answer Choices - VIACs) can be answered algebraically or using the INPUT-OUTPUT approach.

I was about to solving it using the INPUT-OUTPUT approach, but then I saw Mitch post his INPUT-OUTPUT solution.
So, here's an algebraic solution:

We know the TOTAL travel time = t hours
Let's let B = the time spent BIKING
So, t - B = time spent walking.

Bob had a flat tire exactly halfway to school.
Let's start with a word equation.
Distance traveled on bike = Distance traveled by foot
Distance = (speed)(time)
So, we get: (x)(B) = (y)(t - B)
Expand: xB = yt - yB
Add yB to both sides: xB + yB = yt
Factor: B(x + y) = yt
Divide both sides by (x+y) to get: B = yt/(x+y)

So, the TIME spent biking = yt/(x+y)
We can now use this to find the DISTANCE spent biking.
distance = (speed)(time)
So, DISTANCE spent biking = (x)yt/(x+y)
= xyt/(x+y)

Since Bob had a flat tire exactly halfway to school, we know that xyt/(x+y) represents HALF the distance to school.

So, the ENTIRE distance to school = 2xyt/(x+y)

Answer: C

Brent Hanneson - Creator of GMATPrepNow.com

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Sun May 14, 2017 5:39 am

rsarashi wrote:Bob bikes to school every day at a steady rate of x miles per hour. On a particular day, Bob had a flat tire exactly halfway to school. He immediately started walking to school at a steady pace of y miles per hour. He arrived at school exactly t hours after leaving his home. How many miles is it from the school to Bob's home?

A) (x + y) / t

B) 2(x + t) / xy

C) 2xyt / (x + y)

D) 2(x + y + t) / xy

E) x(y + t) + y(x + t)

Here's a different algebraic solution:

Let d = the TOTAL distance to school.

Bob had a flat tire exactly halfway to school
So, d/2 = distance spent biking
and d/2 = distance spent walking

We can write: (time spent biking) + (time spent walking) = t
time = distance/speed
We get: (d/2)/x + (d/2)/y = t
Simplify: d/2x + d/2y = t
Find a common denominator of 2yx to get: dy/2yx + dx/2yx = t
Combine terms: (dy + dx)/2yx = t
Multiply both sides by 2yx to get: dy + dx = 2xyt
Factor: d(y + x) = 2xyt
Divide both sides by (x + y) to get: d = 2xyt/(x+y)

Answer: C

Brent Hanneson - Creator of GMATPrepNow.com

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by Scott@TargetTestPrep » Fri May 19, 2017 5:10 am

rsarashi wrote:Bob bikes to school every day at a steady rate of x miles per hour. On a particular day, Bob had a flat tire exactly halfway to school. He immediately started walking to school at a steady pace of y miles per hour. He arrived at school exactly t hours after leaving his home. How many miles is it from the school to Bob's home?

A) (x + y) / t

B) 2(x + t) / xy

C) 2xyt / (x + y)

D) 2(x + y + t) / xy

E) x(y + t) + y(x + t)

''We are given that Bob bikes at a rate of x miles per hour and walks at a rate of y miles per hour. If we let the distance between his home and school be d, then the distance he bikes is d/2 and the distance he walks is d/2.

Thus, his biking time is (d/2)/x = d/(2x) and his walking time is (d/2)/y = d/(2y). Since he spent t total hours biking and walking:

d/(2x) + d/(2y) = t

Multiplying the entire equation by 2xy, we have:

dy + dx = 2xyt

d(y + x) = 2xyt

d = 2xyt/(x + y)

Answer: C

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